This invention relates generally to broadband communication and telecommunication systems and, more specifically, to methods and systems for the transmission of multiple digital and/or analog input signals on a single high-speed electronic or optical channel.
Modem broadband communication systems often use a single channel to transmit data that is generated by several input sources. These systems may be used in a wide range of applications, including the transport of signals in a bidirectional cable television (CATV) plant and the high speed transfer of data in a network of computers and/or peripheral devices. Cable television systems, for example, use fiber optic cables excited by single-frequency laser transmitters to transfer information between the headend station and primary and/or secondary hubs. Fiber optic links are also used to connect each of the hubs to nodal stations (nodes) that distribute headend-generated information to individual subscribers in the form of radio frequency (RF) electronic signals. In many cases, the information transfer between the headend station and individual subscribers is bidirectional. Conventionally, wavelength division multiplexing may be used to minimize interference between the forward and reverse signals. Systems utilizing this technique assign different wavelength bands to forward and reverse communications in the radio-frequency (RF) portion of the network. Bidirectional optical links use laser transmitters operating at different wavelengths to eliminate crosstalk between counterpropagating signals.
In most cases, it is desirable to transfer several different types of information between the headend and nodes. In the forward and reverse directions, different information types will have different modulation formats (QAM, FSK, On/Off keying, etc.), with each type assigned to a specified frequency band in the transmitted signal. Practically, this is accomplished by mixing each input signal with an intermediate frequency (IF) carrier having various mixing frequencies and summing the resultant signals. The sum is then provided to the transmitter input port. At the receiver, bandpass filters may be used to separate the different signal types, each of which is provided to an individual receiver used to recover the input information.
In an improved digital transport architecture, optical links from the nodes to the headend utilize an on/off keying modulation scheme. In order to transmit different types of information, prior art transmitters must first determine the modulation format of the input data. Once identified, slices of the input bit streams are assembled into output data blocks. Different types of data are assigned to time slots that are delayed by a predetermined time interval with respect to the leading edge of a block. Hardware for accomplishing this task, however, is complicated and expensive.
Thus, what is needed is an improved, low cost method and system for transmitting dissimilar data types along a single broadband communication channel using various, and perhaps arbitrary, modulation formats.